With the increased use of mobile terminal devices such as mobile phones, mobile terminal devices that can be used across borders, namely, multiband radio communication terminal devices have also been actively developed to enable global roaming. Although there has been a trend toward global standardization of frequency allocation, it is being put into practice only gradually. When a frequency band adopted by each nation is seen, some frequency band is allocated as a transmission band in some areas whereas it is allocated as a reception band in other areas. Further, there may arise, in response to the trend toward standardization, a need to accommodate a plurality of bands in the transition phase. Meanwhile, although a band allocated within a nation is relatively wide, there usually exist a plurality of telecommunications carriers that share the band within the nation. Thus, in practice, some percentage of the bandwidth of the allocated band is assigned to the individual telecommunications carrier as an available frequency bandwidth. Accordingly, terminals have only to be operative over only the frequency band assigned to the individual telecommunications carrier. Meanwhile, when roaming overseas, roaming terminals are demanded to be operative over as wide a band of frequencies as possible because such terminals are desired to be operative in as many areas as possible.
There is another context that a power amplifier (hereinafter referred to as “PA”) have properties that its current, distortion characteristics, gains, and the like can greatly vary in response to load fluctuation. An antenna of a mobile terminal device is used while being carried by a user. Thus, the impedance of the antenna can easily change under the influence of the human body and the like.
In order to solve such a problem, an isolator is typically provided at the output stage of the PA so as to isolate the impedance fluctuation and thus to stabilize the load of the PA. Nowadays, however, there have been an increasing number of mobile terminal devices without isolators with a view to reduce the cost and size of the devices, and such devices have been proposed to cope with the aforementioned degradation of properties (see Patent Document 1).
According to the technique described in Patent Document 1 below, a variable load is connected to a signal line between an output terminal of a power amplifier and an antenna element of a mobile phone. Upon fluctuation of the load, the position of the operating load point after the fluctuation on the Smith chart is estimated by detecting a change in operating current or gain of the power amplifier, and then a control circuit performs switch-control of the variable load in a direction to relieve the load fluctuation.    Patent Document 1: JP Patent Publication (Kokai) No. 2000-295055 A